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A279551
Number of length n inversion sequences avoiding the patterns 000, 010, 110, and 120.
22
1, 1, 2, 4, 10, 27, 79, 247, 816, 2822, 10158, 37875, 145695, 576288, 2337412, 9698820, 41089107, 177424188, 779699793, 3482575169, 15791709187, 72621800171, 338388714955, 1596314968112, 7618218238583, 36756086159343, 179176803145900, 882002961543492
OFFSET
0,3
COMMENTS
A length n inversion sequence e_1e_2...e_n is a sequence of integers where 0 <= e_i <= i-1. The term a(n) counts those length n inversion sequences with no entries e_i, e_j, e_k (where i<j<k) such that e_i <= e_j and e_i >= e_k. This is the same as the set of length n inversion sequences avoiding 000, 010, 110, and 120.
LINKS
EXAMPLE
For n=3, the inversion sequences are 001, 002, 011, 012.
For n=4, the inversion sequences are 0011, 0012, 0013, 0021, 0022, 0023, 0112, 0113, 0122, 0123.
KEYWORD
nonn
AUTHOR
Megan A. Martinez, Dec 15 2016
EXTENSIONS
Typo in comment corrected and a(10)-a(27) added by Alois P. Heinz, Feb 22 2017
STATUS
approved